Fluid flow, metasomatism and deformation in the Livingstone Fault at Mt. Raddle, South Westland

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Abstract:

At Mt. Raddle in the Olivine Range, South Westland, pristine mantle harzburgite of the Dun Mountain Ophiolite Belt (DMOB) shows a complete transition through to the serpentinite mélange of the Livingstone Fault, which juxtaposes the ultramafics against the mainly quartzofeldspathic Caples Terrane. The mélange comprises a ~100 m wide zone of scaly serpentinite with embedded pods of massive serpentinite and serpentinised harzburgite. The orientation of the scaly fabric, the long axes of entrained pods and the strike of fractures within the pods are all sub-parallel to the mélange boundaries. As the western mélange boundary is approached, progressive hydration of pristine harzburgite in the DMOB occurs by serpentinisation of olivine and pyroxene, and the development of Fe-chromite from Cr-spinel. Incipient hydration may be recorded by overgrowths of secondary olivine around pyroxene.
A talc-tremolite-clinopyroxene metasomatic reaction zone developed at the serpentinite mélange-Caples Terrane contact. Petrographic and microstructural observations indicate that talc was formed as the initial reaction product. Later production of tremolite and clinopyroxene occurred by replacement and overgrowth of talc, to the extent that talc now comprises <5 vol% of samples derived from the reaction zone. Broadly synchronous growth of tremolite and clinopyroxene indicates that the composition of fluids in the reaction zone was spatially and temporally variable.143Nd/144Nd and 87Sr/86Sr isotope ratios show that fluids interacting with the Caples Terrane were important in controlling metasomatic reactions and are a likely source for Ca in the reaction zone. Isotope profiles indicate that extensive fluid circulation occurred across the full width of the serpentinite mélange. In contrast, serpentinised harzburgites in the DMOB retain primitive (mantle) isotopic ratios indicating that fluids in the mélange did not penetrate significantly into the serpentinising harzburgite.
Early formation of frictionally-weak talc is interpreted to have localised strain in the metasomatic reaction zone. Subsequent formation of tremolite and clinopyroxene, which are stronger and more frictionally unstable than talc, may have promoted strengthening of the reaction zone and a rheological transition towards unstable rupture. The lithologies and sequential metasomatic reactions described here, together with broad estimates of P-T conditions, suggest that the Livingstone Fault at Mt. Raddle provides a superbly exposed example of some of the physio-chemical processes thought to be important along active serpentinite-bearing shear zones, including the San Andreas Fault and subduction zone megathrusts.